Page 197 - Basic Well Log Analysis for Geologist

LOC PS TLARPRETATION CASE STUDIES

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Case Study 4 Answer
The upper Morrow sandstone in the well occurs from doubts about the validity of the 70% water-cut data point.
7,440 to 7.452 ft and ts shown on the SP curve (Fig. 85) by Therefore, you assume a water production amount of
its leftward deflection from the shale baseline. The between 20 to 40%.
permeable nature of the Morrow is secn by the separation Finally, your calculations of the moveable hydrocarbon
between the short normal (R;) and induction (R,) resistivity index (S,/S,,) and moveable oil saturations have favorable
curves. Separation between these curves indicates invasion values.
has taken place. Porosity in the sand from 7,440 to 7,452 ft You have completed your log assessment and are
is apparent by the high (70 to 81 jesee/ft) interval transit encouraged by evidence of good porosities on the sonic log
time (At) on the sonic log (Fig. 86). and good indications of permeability on the resistivity log.
Grain sizc, determined from a plot (Fig. 87) of water Also, you are aware that the low cost of a re-entry well
saturation (S,,) versus porosity (db), is very fine-grained. enhances the economics of the prospect.
This means high water saturation and bulk volume water However, you weigh the positive aspects of the prospect
values. However, because of the very fine grain size, higher against the relatively thin zone and the few data points
volume water valucs (Fig. 88). Data points on the bulk conclusions based on such limited data. Also, fine-grained
than normal values can be tolerated and the well mav still which you have used. The lack of points calls into question
the statistical accuracy of the information, and in turn, your
not produce much water.
As suspected, water saturation (Sy,) values (work Table
D) are high, ranging from 41 to 62%, and so are bulk
reservoirs do not have the permeability of coarser grained
volume water crossplot exhibit only minor scatter from the reservoirs. And, you have determined a 20 to 40% water
production from the well.
hyperbolic line, suggesting the reservoir may be at or near You estimate recoverable oil as 78,000 stock tank barrels
irreducible water saturation. But, the plot (Fig. 88) has only (STB). An estimated investment return is: stock tank barrels
three points. not enough to firmly establish whether or not (STB) X product price X net revenue interest (NRI) = total
the well will produce some water. cost = investment return (78,000 X $32.00 x 0.8125 +
Permeability (Fig. 89) varies from less than one to $275,000 = 7.4:1 return). This 7.4:1 figure is an excellent
approximately 15 md. These are fairly low permeabilities, return on investment and your company accepts your
and probably occur here because the upper Morrow is very recommendation to re-enter the well.
fine-grained. The upper Morrow sandstone was perforated from 7,443
Data points on the relative permeability to water crossplot to 7.451 ft. Initial production flowing (IPF) was [00 barrels
resolution of logging tools along bed boundaries, cast months, production stabilized at 50 BOPD. Two years after
(Fig. 90) increase in value from 0.04 to substantially over of oil per day (BOPD) and 6 barrels of salt water per day
0.1. Two of the three points. however, plot under 0.1.
(BSWPD) with an oil gravity of 38.3°. During the first four
Because permeability relative to water is above zero, some
months, production averaged 75 BOPD, and after six
water will certainly be produced, but probably not in any
great amount.
completion, an offset well located 1,320 feet away, was
drilled. The new well had an initial production flowing
A percent water-cut plot has two points between 20 and
40% water-cut. The third point plots around 70% and is
(IPF) of 336 BOPD and no water. Production came from a
19 foot upper Morrow sandstone reservoir which was,
from a depth of 7,444 ft at the end of the sand interval. The
70% point may not be reliable. Problems, caused by the
structurally, 13 feet high to the original well.

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